The present invention relates generally to through-lamp ground fault current limiting circuits for electronic ballasts, and also to end-of-lamp-life sensing circuits for electronic ballasts.
More particularly, this invention pertains to a through-lamp ground fault current limiting circuit for a non-isolated electronic ballast operable to limit a through-lamp ground fault current caused when one end of a lamp is connected to the electronic ballast and the other end of the lamp is connected to a grounded person by sensing a low frequency ac voltage or a dc voltage generated by the ground fault.
Section 22 of Underwriters Laboratories Standard UL 935 requires that non-isolated ballasts include some sort of through-lamp ground fault current limiting circuit in order to reduce the risk of electric shock for users of such ballasts. Ground faults occur when a grounded person comes into contact with the pins at one end of a linear fluorescent lamp when the other end of the lamp is inserted in a lamp socket that is wired to an energized ballast. When a ground fault occurs, current flows from the ballast, through the fluorescent lamp and the grounded person, to ground. If the ballast does not include some type of current limiting circuit, the ballast may supply enough current to deliver a harmful shock to the grounded person.
As a result of this requirement, through-lamp ground fault current limiting circuits for electronic ballasts are well known in the art. For example, U.S. Pat. Nos. 6,051,940, 5,436,529, 4,893,059, 4,943,886, 5,363,018, and 4,939,427 all teach circuits that may be used to limit through-lamp currents caused by ground faults.
U.S. Pat. No. 6,051,940 teaches a circuit that may be used to limit through-lamp currents caused by ground faults in an isolated electronic ballast having a output transformer. The circuit operates by sensing the increase in the output voltage of the ballast whenever one or more lamps are disconnected from the ballast. When the output voltage of the ballast exceeds a predetermined limit, the circuit generates a signal that is used to reduce the output voltage of the ballast. As a result, the amount of current flowing through the lamp in response to a ground fault is limited. This circuit is designed to be used with an isolated electronic ballast having an output transformer and, accordingly, may not be used with a non-isolated electronic ballast that does not include an output transformer. In addition, this circuit does not actually sense when a ground fault occurs, but rather, senses when a lamp is disconnected from the ballast.
U.S. Pat. No. 5,436, 529 teaches a method of limiting through-lamp currents caused by ground faults in an electronic ballast by using a circuit to sense the conductive path provided by lamp filaments of a lamp connected to the ballast. If a lamp is disconnected from an operating ballast of this type, the series resonant tank circuit becomes unloaded. As a result, the output voltage of the ballast rises to a high value and a sensing circuit, designed to sense this high voltage, causes the ballast to shut down. Depending upon which of the circuits taught by the patent is used, the ballast will not restart until the filament sensing circuit senses that some or all of the lamp filaments have been re-connected to the ballast.
The circuit shown in FIG. 3 of the '529 patent senses all of the lamp filaments, but it should not be used with certain kinds of electronic ballasts and lamp connections. For example, American National Standard ANSI C82.11-1993, paragraph 7.7.2 requires that when electronic ballasts operate lamps connected in series, and the filaments are heated by a common winding, the filaments must be connected in parallel. In addition, since one pair of wires supplies both filaments in ballasts that comply with this standard, electrical continuity cannot be used to determine the presence of the filaments. Thus, the circuit shown in FIG. 3 of the '529 patent cannot be used for ballasts with series-connected lamps built according to the ANSI C82.11-1993 standard because two of the filaments are wired in series instead of being wired in parallel.
Furthermore, when filaments are connected in parallel, one may not be able to simply measure the resistance between the pair of wires connected to the filaments to determine whether both filaments are present. Lamp filaments for T8 linear lamps are not standardized, and they vary from manufacturer to manufacturer, so one may not be sure whether one or two lamp filaments are connected in parallel to a pair of ballast output terminals by simply measuring the resistance between the pair of terminals. In other words, it is possible for one to measure the same resistance between the pair of terminals when only one filament is connected and when two filaments are connected in parallel.
U.S. Pat. No. 4,893,059 teaches a method of limiting through-lamp ground fault currents by using current sensing transformers. Current sensing transformers, however, are too expensive for certain applications.
Two other patents, U.S. Pat. Nos. 4,943,886, and 5,363,018, teach a method of limiting through-lamp ground fault currents in an electronic ballast by using a circuit to sense a high-frequency voltage that is developed between the ballast's inverter power supply terminals and earth ground. The circuit must be connected to earth ground in order to operate properly. If the system is not connected to earth ground, the circuit will not function properly.
While most electronic ballast include earth grounds, occasionally, a person installing the electronic ballast may forget to connect the ballast to earth ground. In other situations, the installer may connect the ballast to earth ground in such a manner that the ballast appears to be connected to earth ground but it is not. This may occur when the installer fails to tighten the connection to earth ground properly. Thus, there is a need for a ground fault detection circuit that will operate properly without a connection to earth ground.
Finally, U.S. Pat. No. 4,939,427, teaches a method of using a circuit to sense the high-frequency voltage developed during ground faults using an additional winding on a common-mode inductor that is part of an electromagnetic interference (EMI) filter. This solution, however, degrades the performance of the EMI filter because the additional winding introduces noise into the EMI filter. This occurs because the additional winding must be connected to circuits that generate a substantial amount of RF noise. Thus, there is a need for a through-lamp ground fault current limiting circuit that does not degrade the performance of the EMI filter.
What is needed, then, is a through-lamp ground fault current limiting circuit operable to sense ground faults without degrading the performance of an EMI filter, operable to function properly without a connection to earth ground, and capable of operation with lamps having parallel-connected filaments.